If you download the MEF source code from the CodePlex site (at http://mef.codeplex.com/SourceControl/list/changesets, link on the right), you will find what you're looking for.

The projects of interest are /oob/sample/Microsoft.Composition.Samples.FabrikamBooks, which is an MVC4 app using Microsoft.Composition, and
/oob/demo/Microsoft.Composition.Demos.Web.Mvc, which is the necessary integration code including an IDependencyResolver and some additional features.

Hope this helps; feel free to shoot some follow-up questions if you need more info!

You can use the CompositionProvider.AddAssembly() and .AddAssemblies() static methods to user other assemblies. Classes in those assemblies under a "Parts" sub-namespace will be picked up just like those in the Parts folder under the web project.

I am also interested in a 'portal framework' example. In particular, I would like to be able to package MVC Areas into separate assemblies and inject them into a portal framework at run-time (think WebParts for MVC). The examples that are out
there using the older MEF are very complex and incomplete. Do you think that this approach is possible or are the routing issues too complex?